Commercially available FTIR spectrometers include movable components due to their mode of operation, so that the operation of such a spectrometer under adverse environmental conditions, in particular temperature fluctuations or vibrations, is possible only by complex and expensive compensation techniques. Dispersive spectrometers typically operate without moving parts and therefore exhibit greater robustness than FTIR spectrometers at a lower cost. However, dispersive spectrometers exhibit, by virtue of their mode of action, i.e., dispersion into wavelengths and the need for an entrance slit, a significantly poorer signal-to-noise ratio in the longer wavelength infrared range, i.e., at wavelengths greater than 3 microns. Between these two extremes, FTIR spectrometers on the one hand and dispersive spectrometers on the other, stands the static FTIR spectrometer, which operates on the principle of an FTIR spectrometer, but has no moving parts. In classical FTIR spectrometers, information is acquired over time in the form of an interferogram, whereas the interferogram is generated in the static FTIR spectrometer via the location on a detector array. Thus, the advantages of FTIR spectroscopy can be combined with the advantages of dispersive spectroscopy. The result is a robust spectrometer with a high signal-to-noise ratio in the infrared range. In addition, since the interferogram in the static structure is immediately available on the detector, very fast measuring speeds are, in principle, possible.
However, due to the spatial expansion of the interferogram and the current line detector technology in the infrared wavelength range, the present static FTIR spectrometer does not achieve the performance of a classical FTIR spectrometer in terms of signal-to-noise ratio and spectral resolution.
The present disclosure describes a new static FTIR spectral apparatus that operates independently of the expansion of the light source and therefore has a high light throughput. It is characterized by the fact that it shows a much lower light loss in the spectral apparatus and a lower material consumption than existing comparable designs. At the heart of this structure is the double use of the beam splitter, once for splitting the infrared radiation into two partial beams and then for imposing a constant optical path difference due to the high refractive index of the beam splitter.
In principle, the applications of the present static FTIR spectrometer are comparable to those of a conventional FTIR spectrometer, as long as the lower signal-to-noise ratio and the lower spectral resolution of the static FTIR spectrometer can be tolerated. Due to the low use of materials and the static concept, the advantages of the disclosure emerge especially under adverse environmental conditions. Thus, the present disclosure can be used, for example, for monitoring production processes in which classic FTIR spectrometer technology is not yet economically feasible. In addition, use in fiber sensor technology for reading out fiber Bragg gratings with light sources of low intensity is possible.
Robust spectrometers in the mid-infrared range (in the wavelength range from 3 μm to 20 μm) can be found in the related art, inter alia, in the three groups described below. Further types of spectrometers for the mid-infrared range will not be discussed here because their technical implementation differs significantly from the present disclosure.
The first group are the time-modulating FTIR spectrometers, which, due to special techniques, such as, double pendulum interferometers according to [1], or retroreflectors according to [2], are robust against vibrations and temperature fluctuations, but also have a higher complexity. The excellent performance in terms of spectral bandwidth, resolution, and signal-to-noise ratio associated with the use of the Fourier transform principle makes this type of spectrometer versatile.
On the other hand, there are many dispersive techniques (grating spectroscopy, linear variable filters, bandpass systems, . . . ) in spectrometers in the mid-infrared range, whose robustness is increased without much additional effort due to the lack of moving parts. Due to the immediate availability of the spectrum on the detector, they are usually characterized by high measuring speeds. In general, these spectrometers are far less complex in construction than robust time-modulating FTIR spectrometers, but they are significantly inferior with regard to performance in terms of resolution, spectral bandwidth, and signal-to-noise ratio.
The third group of robust spectrometers in the mid-infrared range, the static FTIR spectrometers, represents a compromise between both above-mentioned principles of time-modulating FTIR spectrometers and dispersive spectrometers. Here, by skillful arrangement of optical components, the infrared radiation to be measured with a line detector is brought to interference so that a subsequent Fourier transformation of the detector signal leads to the desired spectrum. The interference signal generated in time-modulating FTIR spectrometers over time, the so-called interferogram, is thus generated by spatial expansion and recorded by a line detector, as a result of which high measuring speeds become possible.
By eliminating moving parts, static FTIR spectrometers are more robust and less complex than their time-modulating counterparts. While they are clearly inferior in terms of resolution, spectral bandwidth, and signal-to-noise ratio due to technical limitations of the line detectors to the time-modulating FTIR spectrometers, they achieve a higher light throughput or a better signal to noise ratio through the utilization of the Fourier transformation principle than dispersive spectrometers. Since the present disclosure can be assigned to the group of static FTIR spectrometers, the related art of this particular spectrometer group will now be described in more detail below.
It is a characteristic of all static FTIR spectrometers that they generate the interferogram necessary for spectral computation by spatial expansion. This is done in the related art on the one hand by tilting or providing staircase-shaped mirrors in classical Michelson or Mach-Zehnder interferometers [3], by double-mirror interferometers [4], or by miniaturized static lattice grating interferometer [5]. However, all of these concepts have a dependence of the visibility of the interferogram on the radiating surface of the light source [6]. Since the large radiating surfaces required in the above concepts (due to the low power densities in the mid-infrared range) already lead to a disappearance of the interferogram, these techniques are not suitable for spectroscopy in this wavelength range when using uncooled detectors.
On the other hand, interferograms can be created by spatial expansion using birefringent materials. In this case, the incident light is split into two orthogonal directions of polarization and then brought to interference. Thus, static Fourier transform spectrometers with a large angle of incidence can be constructed [7]. However, the necessary birefringent materials are not transparent in the mid-infrared range, so it is not possible to use this technique in this wavelength range.
Two static spectrometer designs suitable for the mid-infrared range are the static common-path interferometer [8] and the static modified Mach-Zehnder interferometer [6, p. 1461]. Since these concepts differ the least from the present disclosure, an overview of both designs is given in FIG. 1 and FIG. 2. For better illustration, the uncollimated light rays in the set-up are illustrated only by the respective focal point beam.
In both cases there is, in principle, no dependence of the visibility of the interferogram on the radiating surface of the light source used. However, the static modified Mach-Zehnder interferometer is much more difficult to adjust than the static common-path interferometer. As variants of the static common-path interferometer, a complete crystal integration of this spectrometer concept [9] as well as a combination with concave mirrors [10] are published.